+1 on the Dutch oven. An 8 or 10 inch Dutch oven, with a parchment paper liner that you can get to use in them, and a standard box cake or scratch cake recipe, will give you a cake to remember. There'll be a learning curve on the front end of the experience but on the back end you'll have a new cooking utensil and a truly outdoorsy cooking method. Oh, and the Dutch oven can be part of his birthday present!
Here's a temperature chart to get you started. Each size oven has two lines. The first tells how many chunks of charcoal to start, and the second tells how many of that number to put under the oven and how many on the lid.